This invention relates to a system for combining an image, for projecting back-illuminated masks onto a film plane and/or a viewing plane of a microscope.
U.S. Pat. No. 4,057,318 discloses a technique whereby markings on back-illuminated masks are reflected into a microscope. The markings comprise transparent areas on an otherwise opaque film, these areas taking the form, for example, of ruled grids, numerals, scales, and similar symbols, so that these markings appear as bright features on the bright image of the object. The markings are illuminated with a so-called Kohler-type illuminating unit in a manner such that an image of the filament of the illuminating lamp is formed as complete as is possible, via a light collector, in the entrance pupil of the microscope. The image of the marking lies in front of the collector, covering the entire film field or viewing field of the microscope, the size of these fields being dependent on that of the entrance pupil of the microscope.
Since the marking cannot be larger than the light collector, and since, for technical reasons, the collector cannot be made to any desired size, the marking, which generally covers only a portion of the mask, must be very small. This causes difficulty in manufacture. Manipulation of the masks, for example by shifting or rotating them, or by exchanging them, is also an extremely difficult operation which is not simple to carry out since the movements involved are also necessarily very small.